The present invention relates to a tuyere of the type adapted for the injection of gases into a metallurgical vessel and which is characterized by a closely predetermined gas flow rate therethrough when in use.
The use of tuyeres in metallurgical vessels, such as a metal refining furnace or a molten metal container, for submerged blowing steel-making processes, is well known. For example, in the conventional argon oxygen decarburization (AOD) process for making stainless steel, a number of tuyeres are inserted into an AOD vessel. Each such tuyere comprises inner and outer concentric tubes, and such that one gas may be injected through the inner tube, and an auxiliary or shroud gas may be simultaneously injected through the annular gap formed between the tubes. Normally, oxygen or an oxygen mixture flows through the inner tube, and argon or other inert gas, flows in the annular gap formed between the concentric tubes. Depending on the vessel size, there can be one to seven tuyeres, or even more, in each AOD vessel.
As is known, control of the gas flow and the uniform distribution thereof into the vessel are extremely important, not only for enhancing the efficiency of the steel-making process, but also to reduce non-uniform erosion of the vessel walls and the tuyeres themselves. An uneven distribution between tuyeres within the vessel results in non-uniform erosion of the vessel lining and the discharge ends of the tuyeres, and it increases the likelihood of the need for premature replacement of the vessel lining.
The gas control system for the tuyeres in the above described conventional AOD process typically has one flow or pressure controller feeding a common manifold to which the several tuyeres are connected. The piping and manifold system can be designed to produce equal distribution of gas to each tuyere, but each tuyere is the final restriction in the piping system, and the flow characteristics in the tuyeres must be equal in order to obtain equal flow through each tuyere, when connected to a common manifold. Using the present tuyeres of the concentric double tube type, it is very difficult to achieve a predictable gas flow characteristic in the annular gap, in view of the difficulty in achieving manufacturing uniformity. More particularly, the center tube is normally made of copper, and the outer tube is normally made of stainless steel. Once assembled, the outer tube must be precisely concentric with the inner tube to provide a uniform flow characteristic in the annular gap. In present tuyeres, this concentric relationship is maintained by depressions or dimples which are formed in the outer tube and so that the dimples extend inwardly to contact the inner tube and to thereby guide and support the outer tube in a concentric arrangement with the inner tube. As will be apparent, the manufacturing tolerances which must be closely controlled for maintaining a precise uniform flow characteristic between the tubes includes not only the size and shape of the dimples, but also the tolerances of the outer diameter of the inner tube, and the inside diameter of the outer tube. In practice, the additive effect of these variables often produces an undesirable flow variation of plus or minus between about 4% to 6%, or even greater, among tuyeres of like construction.
Flow variations in the bore of the inner tube can be held to a tolerance of plus or minus 2% for a constant tube length, which is usually considered acceptable for AOD processes, by using high quality and dimensionally accurate stock. However, such stock is relatively expensive.
A further aspect of the problem of maintaining a relatively constant and predictable gas flow rate through the tuyere is the fact that the refractory lining of the steel vessel and the discharge end of the tuyere erode during normal usage, thereby shortening the length of the annular gap and the bore of the inner tube of the tuyere. Such shortening in turn reduces the resistance to the gas flow, and the rate of flow increases as the tuyere erodes.
It is accordingly an object of the present invention to provide a tuyere of the described type which effectively overcomes the above noted limitations and disadvantages of the present designs.
It is a more particular object of the present invention to provide a tuyere which comprises concentric inner and outer tubes, and which has a calibrated and predictable gas flow restriction characteristic in the annular gap between the two tubes.
It is also an object of the present invention to provide a tuyere of the described type which provides a predictable gas flow rate through the bore of the inner tube, and while permitting the use of relatively inexpensive stock material for the inner tube.
It is another object of the present invention to provide a tuyere of the described type which provides a relatively uniform flow rate characteristic through the annular gap and the inner tube, at a given gas pressure, which flow rate is maintained as the discharge end of the tuyere and adjacent refractory lining of the vessel erode during use, to thereby provide a uniform distribution of the gases throughout the full life of the refractory lining and tuyere.
It is a further object of the present invention to provide a method of calibrating the gas flow restriction characteristics of each tuyere of a group of tuyeres which are to be used together in a common metallurgical vessel, and so that the gas flow rate through the annular gaps of the tuyeres do not vary by more than plus or minus about 2%.